GAMES:
A good game for this party would be a scavenger hunt, it gives you the feeling of mystery. The prize, the thing that you are looking for can be spyglasses, little notebooks, and little pencils. They can be used to complete mini-missions. The scavanger hunt can be organized with morse code so it will take some time to figure out the message and where to find the next enveope. If it is a long scavanger hunt (10-15 minutes), then it can aswell be the only game that you play at your party.
INVITATIONS:
To attract the guests to come to your party, try putting "TOP SECRET" and "CONFIDENTIAL" onto your your invitations. You can make your invitations look like a folder with a top secret note inside which is really a paper with the party information such as where the party is taking place in and at what time it is.
FOOD:
There isn't exactly a specific way to make food for a "Top Secret" party, you can make the food look like whatever you want, unless you want to put in imagination, such as cookies that are designed like magnifying glasses. If you are using these party ideas for a Birthday Party, then the birthday cake can look like a magnifying glass or a question mark.
PARTY BAG: (loot bag)
In your party bags you can include a note in morse code (and a note that shows the code letters and meanings) that says something like "Thank you for coming to my TOP SECRET party" or something else of your choice. THey can also include cheap sun glasses for disguise and cheap magnifying glasses so your guests can play at home. You should also put inside some candy for your guests to enjoy.
ENJOY!